1. Field of the Invention
The present invention relates to a boot strap circuit to supply energy for initial start up of a power supply.
2. Description of the Related Art
Switched mode power supplies are common for providing power to electronic devices, such as computer systems. In a switched mode power supply, AC power from a wall outlet is filtered and rectified to provide an unregulated DC input voltage. A control circuit controls the power supply by constantly switching current through the primary of a power transformer to transfer power to an output circuit. The output circuit develops the appropriate DC voltage levels for the electronic device. The primary switch preferably comprises a metal-oxide-semiconductor field-effect-transistor (MOSFET) as known to those skilled in the art. Several topologies and modes are commonly used, such as a flyback or a forward converter, which may operate at either constant or variable frequency depending upon the design specifications and needs of the electronic device. The present invention is contemplated for all types of switched mode power supplies.
During the initial power up cycle when the power supply is plugged in or turned on, the system is momentarily in a transient state, since a plurality of voltage and current transients are prevalent in the primary of the power supply. Designers must be aware of all potential transient conditions at power up and establish control of the power supply as soon as possible to prevent hazardous conditions or catastrophic failures. The control circuitry for the power supply monitors power supply operation and regulates performance by switching, shaping and transforming currents and voltages, which may, very quickly, become destructive and hazardous if not precisely monitored and manipulated. Because of this requirement, it is necessary to provide power and establish the initial state of the PWM as early as possible.
The control circuitry must initially derive its power from the primary portion of the power supply to meet isolation requirements between the primary and secondary portions of the power supply for safety purposes. An auxiliary or tertiary winding on the power transformer is usually provided for ultimately powering the control circuitry.. However, the auxiliary winding is unable to provide power immediately, since current through the auxiliary winding is controlled by the control circuitry. To solve this dilemma, a power up or "boot strap" circuit is commonly used to provide the initial power to the control circuitry from the DC input voltage source. Once powered, the control circuitry manipulates the primary switch to allow current flow through the primary and auxiliary inductances, where the auxiliary winding eventually provides the stable and efficient operating power for the control circuitry. Thus, the auxiliary winding eventually takes over providing power to the control circuitry once normal operation is established.
The simplest version of a boot strap circuit is a bleed resistor coupled between the DC input voltage and the VCC input of the control circuitry. The DC input voltage rises almost immediately upon power up, so that the current through the bleeder resistor provides the necessary power to the control circuitry relatively quickly. The resistance of the bleed resistor must be small enough to allow sufficient current to the control circuitry for early control. If the bleeder resistor is too large, it may take too long to power up the control circuitry. However, designers usually attempt to use as large a bleed resistor as practical. First, the bleed resistor must not allow too much current to the control circuitry causing failure. More importantly, once normal operation is achieved, the bleed resistor continues to draw substantial current and absorb significant amount of energy. In fact, it is common that a bleed resistor continually absorbs between several hundreds of milliwatts to as much as several watts in some designs. Most of this energy is converted to heat, which is very undesirable. This extra heat is unwelcome since the power supply must convert the same amount of power in a smaller, lighter package, and thus must dissipate heat at an increased rate. Nevertheless, the bleed resistor was a simple and cheap way to achieve the necessary boot strap control, particularly in common applications that already incorporated cooling fans or other heat extraction methods. The bleed resistor also provides a current limit to the control circuitry in the case of unexpected failure of the auxiliary winding.
With the advent of the Energy Star ratings issued by the Environmental Protection. Agency (EPA), it is becoming increasingly desirable to improve the efficiency of power supplies. Designers are continuously attempting to squeeze more power out of physically smaller power supply devices. Thus, designers are always looking for techniques to increase the efficiency of the power supply without substantially increasing the cost or the size of the power supply. Since the bleed resistor causes a substantial power loss, it is desirable to remove it. Nonetheless, boot strap control is vital to achieve safe operation and to prevent a hazardous situation or catastrophic failure.
An alternative to a bleed resistor is the use of a linear preregulator. The UCC 1883 micropower peak current mode controller device manufactured by Unitrode is an example of a device that incorporates a linear regulator. This product combines main power supply control and regulation circuits into one device. Such devices are common in the industry and are quite diverse in their specific design and function. These devices, however, are commonly used and are typically referred to as pulse width modulators (PWMs). The UCC 1883 device is used in conjunction with an N-type depletion-mode pass MOSFET (NMOS), such as the BSS129. The UCC 1883 device, the depletion mode NMOS transistor and various other miscellaneous components are configured to establish a 9.5 volt linear preregulator to supply input voltage to the UCC 1883 PWM from the DC input voltage. The current path of the NMOS transistor is coupled directly between the DC input voltage and the VCC input of the PWM, and its control terminal receives an output from a control regulator provided within the UCC 1883 device. When the gate to source voltage of the NMOS transistor is zero, a low resistance current path is provided between its drain and source. Thus, the NMOS transistor allows current to flow from the DC input voltage to the PWM almost immediately upon power up.
The drain to source current path of the NMOS transistor is effectively turned off when a negative voltage is applied between its gate to source. The UCC 1883 includes an internal voltage reference and a circuit to regulate the VCC input at approximately 9.5 volts by controlling the gate pin of the NMOS transistor. The regulator operates the NMOS transistor in a linear mode until the auxiliary winding develops the appropriate regulation voltage for providing power to the PWM. The auxiliary winding eventually establishes an appropriate, higher voltage than the 9.5 reference so that the internal regulator turns off the NMOS transistor. Thereafter, the NMOS transistor isolates the DC input voltage from the PWM. In this manner, boot strap power is provided at power up and terminated during normal operation. The use of the bleed resistor is not required and efficiency is improved.
It has been discovered, however, that there are several disadvantages in using the UCC 1883 device. First, the UCC 1883 device is useful for some designs, but is not appropriate for all designs. Furthermore, the UCC 1883 device is more expensive than most PWM devices on the market. This may primarily be due to the fact that the UCC 1883 is a specialized part, requiring an extra internal voltage reference and regulator and a separate output pin to achieve the boot strap control. Thus, the added cost and limited capability of the UCC 1883 limits its use for most switching mode power supply applications.
Another primary draw back to the UCC 1883 device is a potential safety hazard. The NMOS transistor illustrated by Unitrode, such as the BSS129, is used to supply power only during initial power up, but eventually shuts down after the auxiliary winding takes over. However, if the auxiliary winding should fail or if its voltage should drop below the 9.5 volt regulation point for any reason, the internal regulator of the UCC 1883 device activates the NMOS transistor and operates it in the linear mode to regulate VCC at 9.5 volts. This situation is clearly not desired. Almost the entire DC input voltage is placed across the NMOS transistor, which typically has a limited voltage rating. Since the NMOS transistor is operated in its linear mode, it represents a resistance and consumes a considerable amount of power, generating substantial heat. Furthermore, the PWM is potentially exposed to the extremely high voltages of the DC input voltage if the NMOS transistor breaks down due to the high power dissipation.
Therefore, it is desirable to provide a low cost and efficient boot strap circuit for use in almost all switching power supplies, while also maintaining desired safety features in case of unexpected failure.